How primer application can make or break your adhesive’s effectiveness

Primer application for the automotive industry

You may not realize it, but adhesives are used extensively on automobiles, responsible for the attachment of everything from body trim and spoilers to windshields, quarter lights, and back light glass. But adhesives depend on primers in order to work properly, and applying primers on automotive body parts and glass presents special challenges, due to the variety of adhesives that can be used and the importance of safety and aesthetics. To meet the strict quality standards for modern-day vehicles, primer must be applied accurately and repeatedly, in the correct amount, and without compromising the aesthetics of the vehicle.

What is a primer?
Primer is a general term for a substance that cleans and prepares a surface for the application of a coating, such as an adhesive or paint. Primers clean the substrate of contaminants, debris, and particles that could cause imperfections in the coating. They can also change the characteristics of the substrate and serve as a bonding layer.


Body primers

Primers used in the automotive industry generally fall into two categories: body primers and glass primers. Body primers are primarily black in color and contain a mixture of solvent and solids. This means the primer must be agitated before and during use – otherwise, the solids can separate from the solvent and cause a blockage in the applicator. If this happens, the correct formulation of primer may not be dispensed and the adhesive may not work properly, which could lead to a catastrophic failure.

When a black primer is applied with a handheld applicator, agitation is facilitated through the normal handling of the bottle. But when an automated or semi-automated fluid application system is used, it’s important to ensure that the system provides a method for agitation. This is why Designetics developed the bottle holder with stir plate.

The bottle holder with stir plate provides an air-tight seal with the primer container and constantly agitates the primer to ensure the solids stay suspended within the solvent, ensuring the primer works properly and avoiding waste. The applicator system’s HMI (human-machine interface) provides a bar graph showing the bottle’s percentage fill. This allows the operator to monitor the level of fluid in the primer bottle, so the need for a changeout can be predicted and completed at the optimum time, without disrupting production.

The primer bottle – or a Designetics-approved HDPE bottle – rests on the stir plate, which has a magnetic impeller contained within its housing. To achieve agitation, the bottle holder with stir plate uses one of two methods. If the bottle has an internal steel ball, the movement of the impeller causes the ball to agitate the primer fluid. Alternatively, if the bottle doesn’t include a steel ball, a magnetic “pill” is placed inside the primer bottle, and the impeller works on the magnetic pill, just as it would a steel ball. The speed of the impeller, and therefore, the rate of agitation, can be set via the applicator system’s HMI.

But ensuring that black primers are sufficiently agitated is only half the battle. Proper application of black body primer is also critical. If not applied correctly, black primer can be difficult – and sometimes impossible – to remove. Misapplication of black primer commonly requires the part to be taken “off-line” for the primer to be removed manually – a significant disruption that reduces productivity, increases scrap rates, and leads to higher production costs.

The bottle holder with stir plate ensures that black primers are properly agitated before and during application.


Glass primers

Applying primers on automotive glass poses special challenges due to the smoothness of the glass and the importance of aesthetics. Glass primer application is a two-step process that involves a clear, solvent-based fluid that is applied first, acting as a cleaner or activator, before a black primer is applied.

The clear fluid should be applied evenly and in the smallest amount possible so that it dries quickly. But it also needs to be applied in an amount that will be effective, according to the instructions provided by the chemical manufacturer and the chemical’s TDS (Technical Data Sheet). The drying time of the clear fluid must allow it to “flash off” (dry) before the black primer can be applied. This means that excessive application of the clear, solvent-based fluid can significantly increase cycle times, since flash-off time is a gating factor in the application process.

For example, if the clear, solvent-based fluid is being applied along a length of glass from point A to point B, the fluid at or near point A should have flashed-off by the time the applicator reaches point B. This ensures the clear fluid is sufficiently “dry,” but still effective, when the applicator is ready to begin applying black primer.

The application of black primer on automotive glass presents different challenges. First, if not applied correctly and accurately, black primer can cause what’s commonly known as “zebra striping” or the “zebra effect” – a condition where there are gaps, or streaks, in the primer coating. Not only is this unsightly, it also compromises adhesion.

When applying black primer, it’s also important to use just as much pressure as is necessary for the fluid and the applicator. Too much pressure during application can result in what’s known as the “snow plow effect.” This happens when the downforce during application is too great, and primer builds on the outsides of the application, leaving streaks in the middle and much greater thickness on the ends. The snow-plow effect can be especially detrimental, as it can lead to delamination of glass bonding.

Applying primers on automotive body parts and glass presents special challenges, due to the variety of primers that can be used and the importance of safety and aesthetics.
Applicators for every job

Just as there’s no “one best” primer, the best applicator depends on a wide range of factors. At Designetics, our regional sales representatives and engineers understand the technical, quality, and aesthetic factors that need to be considered when choosing an applicator for use in the automotive industry. With this knowledge and experience, we can “dial-in” the best applicator solution based on the primer, its viscosity and dry time, and the substrate. We have over 5000 proprietary fluid applicators to meet almost any requirement, and we can design custom applicators and automated systems, so you get a solution that ensures accurate and repeatable delivery of primer.

To learn more about our fluid applicators and automated systems, please contact us. Designetics is focused on providing complete solutions, and we would love to learn about your primer application and help you improve production time, reduce waste, or even move from a manual to a semi-automated or automated fluid application process.



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